Stucco Flashing Details

Sidebar:

Stucco Flashing Details -

Continued

Flashing Details

The areas of a stucco-clad building that are most susceptible
to leaks are windows and doors, sidewall penetrations, and
horizontal surfaces. In each case, proper flashing can prevent
leaks, but all too often the flashing is either poorly
installed or omitted altogether. Standard flashing details are
difficult to come by, so over the years I have developed
flashing systems that combine code-required techniques with
methods of my own that have endured the test of time.

Flanged Windows

Vinyl or aluminum windows with nailing flanges will drain
properly only if the lath and flashing papers are correctly
layered as follows (see ).

Step 1:

Install a strip of
flashing paper horizontally under the opening.

Step 2:

Install vertical strips of
flashing paper at either side, making sure to overlap the
bottom strip.

Step 3:

Apply a liberal bead of
caulk on top of the flashing paper at the edge of the
opening.

Step 4:

Install the window frame
while the caulk is still wet. If you've used enough caulk, it
should squeeze out from behind the flange. Drive the nails in
all the way; never bend them over. A bent-over nail may look
okay now, but it will eventually rip or puncture the flashing
paper, and could interfere with proper layering of the flashing
and water-resistant membranes.

Step 5

: Apply a liberal bead of
caulk across the top window flange, then run a strip of
flashing paper across the top, pushing the flashing into the
caulk. For extra protection, we sometimes install a 26-gauge
metal Z-flashing or water table over the top of the flange.
This flashing extends about 3/8 inch beyond the window corners
to keep water away from the frame, which we have found to be
susceptible to leaks.
When you install the building paper, roll it out
horizontally, starting at the bottom of the wall and working
your way up. Pay attention to the overlaps, and carefully weave
in the flashing material as you go. I like to see 4-inch
horizontal and 12-inch vertical laps.
Window frames should have a drip edge and stucco key at the
outside edge of the sill; otherwise, water can run back under
the sill, behind the stucco, and enter the wall through any
holes in the flashing paper. When installing window frames, be
careful not to jiggle or shift the frame too much. This could
loosen the joints, creating a crack where water can get
through. Also make sure any weep holes are big enough to drain
freely. If the weep holes are too small or can be easily
plugged, hard driving rain water can back up over the inside
edge of the window and onto the sill.

Flashing for Clad Doors

Doors are susceptible not only to water that falls on them
directly, but also to backsplash from dripping eaves and
overhangs. As with windows, each step in the flashing is
critical to preventing leaks.

Step 1:

If the door will be
installed over a wood floor or subfloor, start by placing a
metal pan with soldered corners at the sill, fastened in place
with one nail driven through each of the wall fins into the
edge of the stud (). The fins on the metal pan should overlap
the metal deck-to-wall flashing at the bottom of the wall by a
minimum 6 inches. If space is tight, make sure the sheet-metal
sub carefully caulks the overlap.

Step 2:

Next, install strips of
flashing paper vertically, flush with the sides of the opening.
The strips should be long enough to cover the pan flanges at
the bottom and to extend under the full width of the flashing
paper that will be installed at the header.

Step 3

: Caulk the flashing paper
1/2 inch from the opening, then install the door jamb. Use a
big enough bead so that the caulk squeezes out when you nail
the door flanges. After the door flanges have been attached,
shim and fasten the jambs to the studs using a pair of nails or
screws 4 inches from top and bottom and 16 inches on-center in
between. This will keep movement in the jamb from causing
cracks in the stucco.

Step 4

: Lay a generous bead of
caulk over the flange at the door head, then bed a final strip
of flashing paper in it. Make sure the paper overlaps both the
nailing flange and the side strips of flashing paper.

Plant-Ons

In many stucco installations, windows and doors are trimmed
with built-up details called "plant-ons." Made of either foam
or wood, these extra layers create additional joints and
corners that are prime candidates for leaks. The key to
preventing leaks at plant-ons is to use two pieces of paper.
One piece runs continuously behind the plant-on to protect the
sheathing; a second piece runs from under the window or door
frame over the plant-on (Figure 3). Avoid folds or bends that
will create a dam for water, and be careful not to tear the
flashing paper when covering the ends of the plant-on.

Figure
3. Flashing paper should be used behind wood or
foam plant-ons used to trim openings. The plant-ons
should be completely wrapped with flashing paper, which
must also be properly woven into each course of lath
paper for proper drainage.

Install vertical plant-ons after the bottom plant-on has
been fully flashed. Run one piece of flashing paper behind the
vertical plant-on, and over the horizontal plant-on. A second
strip of flashing paper, used to cover the vertical plant-on,
should also overlap the horizontal plant-on.
Install horizontal plant-ons at window and door heads last,
again making sure the flashing is correctly layered and
overlaps the side plant-ons so that water drains to the
outside, down and away from the framing.
We use a hammer tacker to hold the flashing paper in place
until we are ready to install the lath, being careful not to
put any holes in the paper. We place staples only at the edges
of the flashing paper, where they will be covered by the window
or door frame and caulking. Any holes created by the staples at
the outside edge will be covered by overlapping building paper.
All plant-ons should be installed a minimum of 1 inch from the
edge of the window or door frame so as to allow enough space
for the flashing paper to easily run out and over the plant-on.
If the plant-ons are installed too close to the frame, the
flashing paper may create a groove or trough that will allow
water to pond.

Wall Penetrations

Vents, utility supply lines, and electrical outlets are all
prime candidates for leaks. For all of these penetrations, the
key to a good flashing detail is to provide adequate backing
around the penetration. For electrical outlets, supply pipes,
spigots, and other small penetrations located near studs, use a
two-piece sheet-metal collar (Figure 4).

Figure 4.

To flash
wall penetrations, the author uses a two-piece
sheet-metal collar (illustration), which is caulked
to the fixture, then overlapped with flashing
paper. A one-piece collar works well when the
fixture falls in the center of a stud bay (photo,
left). The integral flanges on attic and exhaust
vents provide enough support for the stucco, but
should still be caulked and carefully overlapped
with flashing paper (photo, right).

Carefully cut the profile of the pipe or outlet out of each
half of the collar, and allow for the two halves to overlap by
at least 2 inches. Nail the collars only at the top so you can
slip flashing paper under the lower halves (you may have to
install extra blocking in the wall for nailing). After caulking
the sheet metal to the outlet or pipe (all copper pipes should
be sleeved in PVC to prevent corrosion), layer flashing paper
over the top and sides of the collar, making sure to overlap
the lower sheet of paper by 4 inches or more.
Where an outlet or pipe falls near the middle of a stud bay,
use a one-piece sheet-metal collar to provide solid backing for
the stucco. Again, caulk around the cutout and nail only the
top of the collar, leaving the bottom flapping so it can
overlap building paper slipped up from below. Overlap
subsequent layers of paper so that they provide continuous
drainage to the outside.
The same flashing principles apply to larger penetrations,
like exhaust vents and attic vents. Most of these fixtures have
integral flanges, so no additional collar is needed. But the
paper must be layered carefully and should be caulked to the
flange. If the paper rips in the wind, it should be
replaced.

Where Fascia Butts a Wall

Another weak spot in the weather barrier is the intersection
of a fascia or eaves return and a wall. Because the roof pitch
directs so much water to these areas, we take extra precautions
against leaks. Whenever possible, we coordinate our flashing
work with the framing crew so that we can apply a full
36-inch-wide strip of Vycor over the wall before the subfascia
and trim boards are nailed in place (Figure 5). The
self-sticking membrane should overlap any lath paper on the
bottom, but should be overlapped by paper on the top and sides,
so that any water spilling onto the wall will drain
downward.

Figure
5. Flashing where roof trim meets a wall should
be done before the framing is complete. The author
protects the wall with a full-width sheet of
self-adhering membrane, which overlaps lower courses of
flashing paper at the bottom, but is itself overlapped
by paper at the top and sides. It's also essential to
install a metal kick-out flashing on the roof to direct
runoff away from the wall.

Equally important is a kick-out flashing at the edge of the
eaves. This metal flashing is really the first line of defense,
because it catches any water coming off the roof and directs it
away from the wall surface (for kick-out detail, see ","
JLC 9/98). As with all other flashing details, the
building paper should be layered properly so that water drains
to the bottom of the wall.

Horizontal Surfaces

Horizontal stucco surfaces, such as balcony railings and pot
shelves at recessed windows, are especially prone to leaks
because they are fully exposed to the weather. Unless they are
carefully detailed, water from repeated soakings or ponding
will eventually penetrate the stucco base and finish coats and
wet the underlying framing. When the wood swells from this
extra moisture, it causes the stucco to crack, which leads to
more leaks, more swelling, and more cracks.
We always inspect the framing before flashing a horizontal
surface to make sure that it slopes 1/4 inch per foot -- more,
if possible -- so that water will drain freely. If the framing
is level or, worse, sloped back toward the building, we won't
touch it till the problem has been corrected.

Figure
6. Leaks at parapets, pot shelves, and other
horizontal surfaces can cause framing lumber to swell
and crack the stucco. To promote drainage, the
horizontal framing must be adequately sloped and should
be fully flashed with a self-adhering membrane. The
stepped slab also serves to contain leaks.

On a properly sloped horizontal surface, we apply a layer of
Vycor adhered directly to the sheathing and extending down over
the edges to overlap any building paper coming up the sides
(Figure 6). If the surface is in front of a window, we run the
membrane under the bottom nailing flange and onto the sill
plate. Similarly, where the horizontal surface meets a wall, we
run the membrane up the wall where it will be overlapped by
building paper. As with all of our flashing, we're careful not
to tear or punch holes in the paper, and we make sure that the
paper is layered so that water will drain away from the
building.
Use as few nails as possible on the horizontal surface, and
caulk or patch every rip, tear, or hole.

Ron Webber

owns and operates
Prime Plastering in Norco, Calif. He has worked as a plasterer
for more than 30 years.